1. Field of the Invention
The present invention relates to an optical recording and playback apparatus.
Furthermore, the detailed construction of the invention is such that the light beams from the plural laser light sources are focused in a range of small diameter of several .mu.m by a lens or the like, and then applied to an optical recording medium in order to record and play back the signal in high density.
2. Description of the Prior Art
In an example of an optical recording and playback apparatus in the prior art, a microscopic laser beam spot is applied to a rotating optical recording disc. The signals in this apparatus are recorded on the optical recording disc with high recording density using energy of the laser beam such that the intensity is modulated by the recording signal.
On the other hand, in order to play back the recorded signal, a laser light with constant intensity is applied to the position of the recorded signal on the optical recording disc, and the recorded signal is played back by the detection of reflected light or transmitted light from the optical recording disc. In that this optical recording and playback apparatus can record with the high density, memory cost per bit is cheap, high speed access is possible, and the optical head is operated on a non-contact basis with the optical recording disc. Furtheremore, in such apparatus, recording and playback are stable and therefore such systems are attracting much attention as a new media for memory storage in an information oriented society.
In an optical recording and playback apparatus in the prior art, two light spots are applied to adjacent positions on the same information track of the disc. In this apparatus, one light spot records the signal in the information track and at the same time the other light spot plays back the just recorded signal to ascertain whether or not the signal is recorded correctly. Furthermore, in Japanese Patent Application No. Sho 59-52607, an apparatus is proposed which has two light spots which are different in shape and are applied to adjacent positions on the same information track. In this apparatus, one light spot erases the previously recorded signal and at the same time another light spot records the new information signal. In this conventional apparatus, two light beams are focused to small spots by the same focusing lens. For example, the first light beam, which is used for recording and playback, is applied to an appointed information track by means of a known tracking control technique. The position of the second light spot is maintained by the mechanical positioning accuracy of the optical system. The second light spot is applied on the same information track as the first light spot with accuracy of position, for example, of .+-.0.2 .mu.m. According to these conventional methods, it is apprehended that the position of the second light spot may move from the appointed information track on account of the slip of the position of the light source for the second light spot or the position of the optical elements passing the second light beam. Especially, these apparatuses are unsteady against a secular change, a temperature change or a shock from the outside of the apparatus. Furthermore, there is a problem as to the shift of the optical head in the radial direction of disc. The well known swing arm configuration is difficult to adopt in an optical recording disc. As to a straight shift configuration, since the above-mentioned accuracy of the light spot position is required both for the inner or outer information track of the optical recording disc, the shift mechanism itself of the optical head for the optical recording disc must be highly precise.